The invention concerns an optical pick-up for reading and/or writing a magneto-optically recording medium, whereby a source of light projects light onto the recorded medium and the light reflected by the medium or penetrating through it is deflected onto one photodetector or another in accordance with the direction of polarization of the light. An optico-magnetic recorded medium, also called a magneto-optic disk, is described in the article "Magnetooptische Versuche dauern an" magneto-optic experiments continue"]on pages 37 to 41 of Funkschau 13 (June 20, 1986).
A magneto-optic disk, in contrast to a conventional compact disk, has impressions. Behind the light-permeable layer is a magnetic layer that data can be recorded: in and read out from. How data are written onto a magneto-optic disk will now be described. The magnetic layer is heated to above the Curie point by a laser beam focused on the disk. It is usually sufficient, however, to heat the magnetic layer to the compensation point, which is just below the Curie point. Behind the focal point on the disk is an electromagnet that magnetizes the area heated by the laser beam in the direction or the other. Since the heated area cools down to below the Curie point again once the laser beam is turned off, the direction of magnetization established by the electromagnet is preserved, "freezing in" in a manner of speaking. The individual bits are accordingly stored in domains of different directions of magnetization. One direction of magnetization for example corresponds to a domain of logical one and the other direction of magnetization to a logical zero.
The data are read out by exploiting the Kerr effect. The plane of polarization of a linearly polarized beam of light is rotated by a measurable angle when the light is reflected by a magnetized mirror. Depending on the direction that the mirror is magnetized in, the plane of polarization of the reflected beam is rotated to the right or to the left. Since, however, the individual domains on the disk act like magnetized mirrors, the plane of polarization of a scanning beam of light will be rotated at a small but still measurable angle to the right or left in accordance with the direction of magnetization of the domain just scanned.
The Faraday effect, which is similar to the Kerr effect, is exploited with magneto-optic disks through which the beam of light is transmitted. The optical pick-up determines from the rotation of the plane of polarization of the light reflected by the disk or transmitted through the disk whether the bit it is sensing is a logical one or a logical zero.
In known optical pick-ups, the light is deflected by a polarizing beam divider onto one of two photodetectors in accordance with its direction of polarization, and a data signal is derived from the difference between the signals from the two detectors.
To ensure that the polarizing beam divider will symmetrically separate light with a plane of polarization that is not rotated due to the Kerr effect or Faraday effect, meaning that each photodetector will receive the same light energy, the divider must be adjusted such that the angle between the unrotated plane of polarization and one lateral edge of the divider is 45.degree. . When the plane of polarization of the light has been rotated in one direction as a result of the Kerr effect or Faraday effect, one photodetector will receive the same light energy as the other if the plane of polarization of the light is rotated at the same angle in the other direction.
This situation will now be explained with reference to FIG. 1, which is a vector diagram.
FIG. 1 illustrates the polarization vector Vl of light with a plane of polarization that has been rotated in one direction as the result of the Kerr effect or Faraday effect. The V2 in FIG. 1 is the polarization vector of the light with a plane of polarization rotated in the other direction. Il and I2 are the vector components that parallel the edges of the polarizing beam divider.
The divider is correctly positioned in the path of the beam when both vectors are symmetrical to the angle bisector V' which is the vector of polarization of the light with the plane of polarization that is not rotated. The Kerr effect or Faraday effect will rotate the plane of polarization of the light out of its midposition V by an angle of either +.phi. or -.phi..
Why the angle between the edges of the polarizing beam divider and the unrotated plane of polarization must be as precisely 45.degree. as possible will now be explained with reference to the following formula. S(t) represents disruptive noise components resulting from the optical properties of the magneto-optic recording medium and from laser noise. The magneto-optic signal MS is obtained from the formula ##EQU1##
Since the plane of polarization is not rotated very far at the angle of +.phi. or -.phi. as a result of the Kerr effect or Faraday effect, cos .phi. is approximately 1. If the polarizing beam divider is positioned in the path of the beam such that angle .alpha. is precisely 45.degree. , disturbance component S(t) .multidot. sin (45.degree. ) .multidot. cos .phi. will cancel out because the term sin (45.degree. -.alpha.) is zero.
Since, however, magneto-optic disks are made out of different materials with different optical properties, different double refractions for example, the prism beam divider can be optimally adjusted for only one type of disk. The disruptive component S(t) .multidot. sin (45.degree. -.alpha.) cos .phi. is not zero in magneto-optic disks with a different double refraction. To keep this component constantly suppressed, the beam divider must be readjusted for each type of disk.